Most literature advocates candling at day 7, 14 and 21. You should try to resist to candle more than this as excessive handling and temperature changes may compromise the developing chick. The term candle originates from when eggs were viewed using candles. Nowadays you can see through the shell with a piece of cardboard with a hole cut into it and a torch. Place the egg on the cardboard and shine the torch underneath it, this will illuminate the egg sufficiently to examine it.
On day 7 you should expect to see spider veins and a dark patch which is the developing embryo and a good air sac at the broad end of the egg.You should discard and eggs that appear to have developed a blood ring (infected eggs) and may explode if left in the incubator and those that are 'clear', no veins or black spot (not fertilised).
From day 14 onwards the egg appears very dark as the developing embryo occupies most of the egg so there is little point in candleing.
Bantam chicks will hatch at around 19 days and I will always candle an egg that has not hatched after 20+ days before I give up on it and discard it.